1. Field of the Invention
The present invention relates to an image forming apparatus configured to form an image on a transfer material in, for example, electrographic printers, copiers, and printing machines.
2. Description of the Related Art
There are a plurality of types, such as an electrographic type, a offset printing type, and an inkjet type, of image forming apparatuses. Hereinafter, related techniques are described by taking an electrographic type color image forming apparatus as an example.
Color image forming apparatuses are classified according to its configuration mainly into either a tandem type in which a plurality of image forming units are arranged side by side, or a rotary type in which a plurality of image forming units are cylindrically arranged. Color image forming apparatuses are also classified according to the employed transfer technique, mainly into a direct transfer type in which a toner image is transferred onto a sheet material from a photoreceptor, or an intermediate transfer type in which a toner image is once transferred onto an intermediate transfer member and in which the transferred image is subsequently transferred from the intermediate transfer member to a sheet material.
FIG. 9 is a cross-sectional view of an image forming apparatus of the intermediate transfer tandem type in which four color image forming units are arranged on an intermediate transfer belt. The image forming apparatus of the intermediate transfer type does not need to hold the transfer material on a transfer drum or a transfer belt, while the apparatus of the direct transfer type should hold the transfer material thereon. Thus, the image forming apparatus of the intermediate transfer type can deal with a broader variety of transfer materials, such as super-thick paper and coated paper. Also, the image forming apparatus of the intermediate transfer type is advantageous in that parallel processing can be performed in a plurality of image forming units and that a batch transfer of full color images can be achieved. Consequently, the image forming apparatus of the intermediate transfer type is suitable for realizing high productivity. Hereinafter, an operation of the image forming apparatus is described below by referring to FIG. 9.
A transfer material S is accommodated by being loaded on a lifting-up unit 52 in a paper feeding apparatus 51. The transfer material S is fed by a paper feeding unit 53 in synchronization with image formation in image forming apparatus 50. The paper feeding unit 53 may be of the type that utilizes friction separation due to a paper feeding roller, or of the type that utilizes separation attachment due to air. The apparatus shown in FIG. 9 employs the paper feeding unit of the latter type that utilizes air in feeding paper. The transfer material S fed by the paper feeding unit 53 passes through a conveyance path 54a of a conveyance unit 54 and is conveyed to a registration unit 55. After skew correction and timing correction are performed on the transfer material S in the registration unit 55, the transfer material S is sent to a secondary transfer unit. The secondary transfer unit is a toner image transfer nip unit that consists of a secondary transfer inner roller 503 and a secondary transfer outer roller 56, which are substantially opposed to each other, and that transfers a toner image onto the transfer material S. The secondary transfer unit provides a predetermined pressing force and an electrostatic load bias thereby to cause an unfixed image to be adsorbed onto transfer paper.
A process of forming an image sent to the secondary transfer at a timing similar to that at which the above-described process of conveying the transfer material S to the secondary transfer unit is performed, is described below. An image forming unit 513 consists primarily of a photoreceptor 508, an exposure unit 511, a developing unit 510, a primary transfer unit 507, and a photoreceptor cleaner 509. The exposure unit 511 emits light to the photoreceptor 508, which has a surface preliminarily uniformly charged by a charging unit and is rotated in a direction of an arrow A shown in this figure, according to an image information signal sent thereto. The light passing through a diffraction unit 512 forms a latent image. Then, toner development is performed on the electrostatic latent image formed on the photoreceptor 508 in this way. Thus, a toner image is formed on the photoreceptor 508. Subsequently, the primary transfer unit 507 provides the predetermined pressing force and the electrostatic load bias to thereby transfer the toner image onto the intermediate transfer belt 506. Thereafter, a small amount of untransferred toner left on the photoreceptor 508 is collected by the photoreceptor cleaner 509. Then, the toner is prepared for forming the next image again. The apparatus shown in FIG. 9 has four image forming units 513, which are constructed as described above and respectively correspond to yellow (Y), magenta (M), cyan (C), and black (Bk).
Next, the intermediate transfer belt 506 is described below. The intermediate transfer belt 506 is stretched by rollers, such as a drive roller 504, a tension roller 505, and a secondary transfer inner roller 503, and is driven and conveyed in the direction of an arrow B shown in this figure. Thus, a process of forming images respectively corresponding to the colors Y, M, C and Bk by the image forming units 513 in parallel to one another is performed at a timing with which each of these images is superimposed on the upstream toner image having been primary-transferred onto the intermediate transfer belt. Consequently, a full-color toner image is formed on the intermediate transfer belt 506 and is conveyed to the secondary transfer roller 56.
After the process of conveying the transfer material S and the process of forming the images are performed, the full-color toner images are secondary-transferred onto the transfer material S in the secondary transfer unit. Subsequently, the transfer material S is conveyed by a pre-fixation conveyance unit 57 to a fixing unit 58. The fixing unit 58 is operative to heat-fix the toner onto the transfer material S by utilizing the predetermined pressing force of the rollers substantially opposed to each other or to the belt and also utilizing heating effects of a heat source, which is usually a heater. Then, one of conveyance paths of the transfer material S having a fixed image obtained in this way is selected by a branch conveyance unit 59. That is, in the case of one of the conveyance paths, the transfer material S is discharged directly to a discharging tray 500. Alternatively, in a case where two-sided image formation should be performed, the transfer material S is conveyed to a reversal conveyance unit 501.
An operation of conveying the transfer material S in the case of performing the two-sided image formation is described below. The leading end and the trailing end of the transfer material S sent to the reversal conveyance unit 501 are interchanged by performing a switchback reversal operation. Then, the transfer material S is conveyed to a two-sided transfer material conveyance unit 502. Subsequently, this transfer material S is joined with a transfer material, which is conveyed from the paper feeding unit 51 in the subsequent job, from a paper refeeding path 54b of the conveyance unit 54 at the right timing. Similarly, the joined transfer materials S are sent to the secondary transfer unit. A process of forming an image on a rear surface (that is, a second side) of the transfer material S is similar to the process of forming an image on a front surface (that is, a first side) of the transfer material S. Thus, the description of the process of forming an image on the rear surface is omitted herein.
As described above, the image forming apparatus 50 employs the switchback method to reverse the transfer material. The switchback method is the most commonly employed method reversing a transfer material because the configuration is simple and is space-saving. However, the switchback method has a drawback in that when image transfer is performed on the front and rear surfaces of the transfer material, a reference for the direction of conveying the transfer material is changed, that is, the leading end and the trailing end of the transfer material are interchanged. As described above, the image forming apparatus configured as illustrated in FIG. 9, is advantageous in high productivity and media supportability. Thus, recently, the image forming apparatus has been usually used for near-print purposes (typically, for print-on-demand applications). In such a case, very high image printing accuracy is demanded. Thus, the registration unit 55 usually has a configuration that is advantageous for skew-correction, and has, for example, a skew roller system. Under such conditions, the presence of different references for the direction of conveying the transfer material, which respectively correspond to the front side and the rear side of the transfer material, is a large obstacle to the achievement of the image printing accuracy, especially, the accuracy of displacement of an end margin in a direction of conveying the transfer material (that is, an auxiliary scanning direction). This is because of minute variations in the dimension of preliminarily cut transfer materials. Thus, as long as the transfer of the toner image on the intermediate transfer belt 506 is performed onto the front surface and the rear surface of the transfer material in the opposite directions, respectively, the end margin varies by an amount of the variation in the dimension of the transfer material even when the transfer of the toner image on the intermediate transfer belt 506 is made to coincide with the formation of the image on the transfer material S in a uniform way. Consequently, a blank part of the image or an additional margin occurs in a cutting process or a folding process. This may cause a quality problem.
To solve such a problem, various related techniques have been proposed to recognize the same reference at the transfers of the toner image onto the front surface and the rear surface of the transfer material, respectively, as described in Japanese Patent Application Laid-Open No. 10-190975. According to a certain related technique, an amount of variation is detected and is corrected by, for example, adding indistinctive dot patterns to the transfer material and then counting the added dot patterns. However, the formation of essentially unnecessary dot patterns on the transfer material results in wasteful consumption of toner. Sometimes, a claim may be made for the patterns themselves.
Therefore, a related method of detecting a reference for the transfer material itself, that is, an edge thereof, has become the norm. As described in, for instance, Japanese Patent Application Laid-Open No. 2003-35974, a detection unit is provided that is adapted to detect a rear end (that is, a front end serving as a reference at the transfer of the image onto the front surface of the transfer material) of the transfer material in the process of interchanging the leading end and the trailing end of the transfer material and then refeeding the transfer material. The position of an end of the transfer material and the timing, with which an image is formed, are calculated according to a detection signal.
Also, in a related technique described in Japanese Patent Application Laid-Open No. 11-237768, a detection unit is provided to detect the leading end and the trailing end of the transfer material. A rear end margin is calculated from positional information on the rear end of the transfer material, which is detected when the image is transferred onto the front surface. Consequently, when the leading end of the transfer material (that is, the rear end thereof detected at the transfer of the image onto the front surface) is detected, the position of the image on the rear surface is set according to the value of the rear end margin.
However, even when the end margins at the transfers of the image onto the front surface and the rear surface are made to coincide with each other, it is actually difficult to obtain the sufficient quality of a print product. This is because the transfer material, onto the rear surface of which the image is transferred, has been provided with the toner image transferred onto the front surface thereof, which is fixed thereto by the fixing unit 58 in the image forming apparatus 50 illustrated in FIG. 9, so that the transfer material contracts in a direction of width of the transfer material which is perpendicular to the direction of conveying the transfer material (that is, a main scanning) and in a direction of conveying the transfer material (that is, an auxiliary direction). There is variation in the contraction of the transfer material, which is caused after the transfer material passes through the fixing unit 58, in the direction of interspaces extending among fibers thereof depending upon the rate of evaporation of moisture contained in the transfer material. Thus, there is the need for providing a unit which is adapted so that the end margins respectively corresponding to the front surface and the rear surface are equal to each other, and that the magnification of the image formed on the front surface is made to be equal to the magnification of the image formed on the rear surface, so as to set exactly the same image position accuracy corresponding to each of the front surface and the rear surface of the transfer material. For example, Japanese Patent Application Laid-Open Nos. 2002-338084 and 2003-241610 describe units adapted to take notice of change in the magnification of each of the images respectively formed on the front surface and the rear surface and to make the magnification of the image formed on the front surface and that of the image formed on the rear surface to be equal to each other.
Generally, in a case where the image position accuracy of a print product is stringently required in, for example, a printing market, it is necessary that the approximate displacement in the auxiliary direction between the images respectively formed on the front surface and the rear surface is ±0.5 mm to ±1 mm. This displacement is caused mainly by mechanical tolerance and by variation due to the transfer material. The former cause may be suppressed to a certain degree by controlling the number and the precision of intervenient mechanical parts. However, it is difficult to directly suppress the latter cause. Conversely, the printing accuracy of the image forming apparatus depends upon how variation due to the transfer material can be suppressed.
Thus, there have been proposed various techniques of estimating the length of the transfer material by utilizing a detection unit adapted to detect the leading end and the trailing end of the transfer material. To actually achieve the aforementioned accuracy of approximately ±0.5 mm to ±1 mm, practical realization of such a unit is difficult, unless the accuracy of detection or estimation of the length of the transfer material is equal to or less than ±0.3 mm. The value ±0.3 mm is an approximate value of variation of expansion or contraction of the transfer material under the same conditions (the kinds, the image, and the environment of the transfer material). In a case where the precision of detection or estimation of the length of the transfer material is less than this approximate value, in order to obtain good image position accuracy, it is better to select a method in which an operator measures the displacement of the image between the images formed on the front surface and the rear surface from an output sample and also inputs a uniform correction value, though this is troublesome.
From this viewpoint, the aforementioned related art is insufficient for achieving the image position accuracy stringently required in the printing market, due to many error factors in detection and estimation of the length of the transfer material. This is because of the facts that a phenomenon of minute oblique passing (that is, the transfer material is conveyed in an inclined posture), strictly speaking, occurs in the transfer material to be conveyed, and that a minute skew (the posture of the transfer material is inclined due to the difference in circumferential velocity between the left and right conveyance rollers) occurs therebetween. Also, the conveyance roller has initial variation in outside diameter and, changes and varies in durability due to wear, so that a difference in conveying speed is caused among a plurality of rollers conveying the transfer material. Thus, a signal outputted by the detection unit includes substantial errors, so that the estimated length of the transfer material deviates significantly from the actual length thereof.
Although the related detection unit can detect timing with which the leading end and the trailing end of the transfer material pass therethrough, this detection unit cannot detect the influence of the oblique passing, the skew, or the difference in the conveyance speed. It has been described that the length of the transfer material and an amount of shift in the timing, with which the image is formed, are calculated according to a detection signal. However, the length of the material and the amount of shift are calculated according to these methods assuming that the speed of conveying the transfer material is an ideal speed. Thus, even in this process, the signal includes errors having significant influence on the accuracy of estimation.